The invention relates to a method for process monitoring during die-casting or thixo-forming of metals in vacuum in a mould.
The automobile industry is placing ever increasing demands with respect to tolerances on the mechanical properties of diets and thixo-formed parts. As a means of achieving these high quality requirements, great importance is attached to the maximum possible monitoring of the process characteristics and their reproducibility. One essential factor, which directly determines the mechanical properties of a part manufactured by die-casting or thixo-forming, is the sequence in which the metal solidifies in the mould.
The object of the invention is to provide a process of the kind mentioned at the start by means of which the manufacture of die cast and thixo-formed parts can be monitored continuously and reliably under production conditions.
The object of the invention is achieved by way of the invention in that the change in temperature as a function of time is measured continuously at one place in the system and the change in temperature of the system is calculated in real time by means of a program and that, from the change in temperature of the system, the change in heat flow as a function of time and from the change in heat flow the change in the energy of the system and the magnitude of the heat of solidification of the metal in the mould is calculated, whereby the values calculated at a given time are used as characteristics for monitoring purposes.
The amount of heat exchanged between the metal to be cast and the mould halves determines the rate of solidification of the part manufactured by die-casting or thixo-forming. As the characteristics of this exchange directly contribute to determining the mechanical properties of the die-casting or thixo-formed part, it is essential that the solidification of the metal in the mould is monitored in order to achieve a high quality standard.
Determining the amount of heat conducted away by the mould makes it possible to detect whether the solidification occurs completely within the mould, whether pre-solidification occurs or what ratio of solid to liquid is present in a thixo-material.
A major fraction of the heat that is exchanged during solidification comes from the latent heat of fusion released during solidification. The magnitude of the latent heat depends in turn greatly on the fraction of liquid metal present on filling the mould. The amount of latent heat released via the mould halves depends in turn on the metal to be cast or on the alloy employed, and can be influenced by the temperature of the mould or mould halves, by the pressure applied, by the speed of the piston and by the thickness of the layer of lubricant.
The exchange of heat occurring during the different phases of solidification is calculated with the aid of a program The calculations are based on temperature measurements on the mould, whereby preference is given to measuring temperatures in the mould wall, and the change in temperature at the shape-giving surface of the mould is calculated. To that end, sensors are employed attached to the walls of the mould halves a distance of e.g. 1 mm from the surface. The program takes into account the inverse conduction of heat and calculates in real time the temperature at the shape-giving surface of the mould halves and the heat exchange between the solidifying metal and the mould. By means of temperature sensors arranged in this manner, it is possible to monitor in real time the uniformity of the cooling process and the thermal equilibrium at the surface of the mould during the successive phases of casting and cooling. The sensors are therefore preferably arranged where the thermal equilibrium and the solidification can be readily registered.
The index i.e. characteristic for the amount of heat conducted away at a given time lies preferably between approx. 20% and 100%, in particular between approx. 50% and 100% of the maximum heat of solidification
In practice it has been found useful to calculate, as die-casing characteristic, the heat of solidification at a given time of 0.1 to 2 s, preferably 0.3 to 0.8 s and, in particular, approx. 0.5 s.
A further characteristic that may be employed is the temperature calculated for the mould surface immediately before each shot.
From the change in temperature it is possible to calculate the change in the heat exchange coefficient. The calculated value of heat exchange coefficient at a given time e.g. the maximum value in the solidification or cooling phase, or the overall plot in values may be employed as further characteristics.
From the change in the energy of the system it is also possible to employxe2x80x94as an additional characteristicxe2x80x94the difference between the energy values at the start of filling the mould on successive shots.
From the change in temperature of the system it is possible to calculate the change in length of solidification as a function of time. By the length of solidification is to be understood the thickness of solidified metal measured from the mould surface. The length of solidification at a given time may be employed as a further characteristic.
Further possible characteristics are the minimum pressure, which is determined from the measurement of the change in pressure in the mould interior, and the minimum relative humidity measured in the mould interior immediately before a shot.
For the purposes of process monitoring the calculated or measured characteristics as actual values can be compared with the intended values, whereby provision may be made for an alarm signal being made when the actual values deviate too much from the intended values within a tolerance range and, when the tolerance range is exceeded, the die-casting or thixo-forming process is interrupted.
The intended value for the amount of heat of solidification removed is specified e.g. as an average value with a standard deviation. The standard deviation may e.g. be fixed as the first tolerance limit, which if exceeded by the actual value causes an alarm signal to be given.
Adhering to the intended values of the characteristics results in a uniformly high quality standard. Deviations from the intended values are registered in real time with the result that the appropriate corrective measures can be taken quickly.
A particularly interesting field of application for the described method is that of die-casting and thixo-forming, in particular as applied to aluminium and magnesium alloys, for example for manufacturing safety parts for the automotive industry.